1. Field of the Invention
This invention relates to a misalignment detection apparatus. For example, in an exposure apparatus for manufacturing semiconductor devices, when sequentially exposing and transferring a fine electronic circuit pattern formed on each of at least two kinds or first objects, such as masks (reticles) or the like, onto a second object, such as a wafer or the like, the apparatus of the invention is suitable as an alignment apparatus for performing relative alignment between the mask and the ware, an apparatus for measuring alignment accuracy of a printed pattern on the wafer after printing the pattern on time mask (reticle) the wafer, or the like.
The present invention can also be widely applied to a position detection apparatus for detecting the position of an object by providing at least one diffraction grating on the object.
2. Description of the Related Art
A method of detecting misalignment of at least one diffraction grating by measuring the phase difference between two beat signals obtained by, causing diffracted light beams from two diffraction gratings to independently perform heterodyne interference has been proposed as a method of detecting misalignment of at least one diffraction grating with high accuracy.
In a misalignment detection apparatus shown in FIG. 1, a laser light beam from a laser 111 is divided into two light beams, which pass through ultrasonic modulators 114 and 118 before being incident upon a diffraction grating MP, comprising two diffraction gratings. Hence, wavefront aberration is produced in the light beams after passing through the ultrasonic modulators 114 and 118, and therefore the wavefronts of the light beams incident upon the diffraction grating MP are distorted. At that time, the phase of an interfered light beam produced from two diffracted light beams is determined by the position of the diffraction grating MP in the x direction and the mean value of phases of the light beam on respective regions cut by the two diffraction gratings. That is, when the position of a diffraction-grating mark with respect to a beam spot (optical system) changes, the phases of two beat signals, serving as misalignment detection signals, change. As a result, an error is produced, causing degradation in the reproducibility of measurement.
In order to reduce the error, the alignment accuracy of the mark (diffraction grating) must be increased. For that purpose, a high-precision alignment wafer stage, a high-resolution mark-position detection apparatus and the like are required, thereby causing an increase in the size of the entire system, and causing problems from the viewpoint of the production cost and the throughput of the entire system.
In addition, since the above-described measurement utilizes the interference of two beams, for example, it is difficult to adjust an optical system, comprising mirrors and the like, for providing a desired incident angle on the diffraction grating.
Furthermore, for example, in an exposure apparatus for manufacturing semiconductor devices, a misalignment detection apparatus having increased resolution is required as the degree of integration of obtained IC's (integrated circuits) increases.
A method of detecting misalignment of a diffraction grating, comprising two diffraction gratings, each serving as a pattern, from the phase difference between two beat signals obtained as a result of independent heterodyne interference of diffracted light beams from the two diffraction gratings has been proposed, for example, in U.S. Pat. No. 4,710,026. FIG. 1 is a schematic diagram of a misalignment detection apparatus in an exposure apparatus for manufacturing semiconductor devices described in the above-described patent application.
In FIG. 1, the laser light beam from the laser light source 111 enters a beam splitter 113 after passing through collimating lenses 124 and 126, and is divided into two light beams. The frequencies of the divided light beams are shifted by frequencies .DELTA.f1 and .DELTA.f2 by the ultrasonic modulators 114 and 118, respectively. The light beams are then reflected by mirrors 115 and 119 and mirrors 116 and 120, respectively, and are projected onto the diffraction grating MP on a wafer 102 from different directions.
As shown in FIG. 2, the diffraction grating MP comprises two diffraction gratings MPa and MPb, which are shifted by .DELTA.x with respect to each other. The light beams are diffracted by these diffraction gratings in a direction perpendicular to the wafer 102. The diffracted light beams interfere with each other, and the obtained interfered light beam is incident upon a half-mirror 105 after passing through an objective lens 103 and a diaphragm 104. The light beam passing through the half-mirror 105 is incident upon a photoelectric transducer 106. The light beam reflected by the half-mirror 105 is incident upon an eyepiece 107, and is used for observing interference fringes.
Respective interfered light beams caused by light beams diffracted by the diffraction gratings MPa and MPb are detected by photoelectric transducers 106a and 106b, which constitute the photoelectric transducer 106, respectively. Since the frequencies of the two light beams incident upon the diffraction grating Mp slightly differ from each other, detection signals representing the interference light beams become sinusoidal beat signals whose frequency corresponds to the frequency difference therebetween.
The phases of the beat signals from the photoelectric transducers 106a and 106b are detected by a misalignment-detection control circuit 125, and the amount of misalignment .DELTA.x is calculated from the phases of the beat signals and the phases of reference frequencies supplied to the ultrasonic modulators 114 and 118.